Force control in hydraulic systems is typically achieved by regulating the pressure of hydraulic fluid in a cylinder under the head of the piston by varying the flow of hydraulic fluid from or to a manifold using a flow-regulating mechanism such as a valve. In many cases, the pistons of the hydraulic cylinders are driven by external mechanisms. This arrangement is shown in a generic form in FIG. 1. One example of such an arrangement is an active or semi-active shock absorber in a vehicle in which the piston of a hydraulic cylinder is actuated by road forces acting on the wheel, and the cylinder is required to transfer a specific force to the vehicle body in order to maintain ride quality. FIG. 2 shows how such a system is essentially configured. Another example of an externally driven system is a hydraulic cushion system used to vary the blank-holder forces in the manufacture of sheet metal stamped parts, in which the configuration includes a set of hydraulic cylinders, which are placed in the tooling or in the bed of the press, and driven by the ram of the press during the stamping operation with the objective of delivering specific forces to the sheet metal blank at specific instances during the operation. FIG. 3 shows an example of such a configuration.
Heretofore, known force-control methods required setting of control parameters based on specific dynamic and kinematic characteristics of the driving mechanism. In particular, the gains of standard proportional-integral-derivative (PID) controllers were set for the specified mechanism, along with other hydraulic components such as pre-filled valves that compensate for various effects such as variable flow-rate. For example, the integral gain of the controller in U.S. Pat. No. 5,339,665 to Yoshikawa is adjusted based on a priori information on the drive-speed of the press, which is manually put in as the number of strokes-per-minute (SPM) into the memory of the controller. Even advanced control techniques such as those described in the publication by S. Ananthakrishnan, S. Agrawal, R. Venugopal, M. Demeri, titled “RCS Based Hardware-in-the-loop Intelligent System Design and Performance Measurement,” Proceedings of PerMIS 2002, NIST, Gaithersburg, Md., 2002, require controller parameters to be set for the driving mechanism. Controllers for systems described in U.S. Pat. No. 6,732,033 of May 2004 to Laplate et al, and the publication by C. Mo and M. Sunwoo, titled “A Semiactive Vibration Absorber (SAVA) for Automotive Suspensions,” Int. J. of Vehicle Design, pp. 83-95, Vol. 29, Nos. 1/2, 2002, are also based on knowing certain parameters such as the mass and acceleration profiles of the driving system. Thus, known externally driven hydraulic force control systems were customized and could not be easily reconfigured for use with a different external driving mechanism. The ability to use a single method and system to achieve force control in hydraulic cylinders driven by a wide range of external mechanisms provides a significant improvement in the state-of-the-art by widening the range of applications of such systems. For example, reconfigurable blank-holder force variation systems can be developed for use in a variety of presses, tools and dies, with as many cylinders as required, and these systems will not require recalibration of control parameters and customization of hydraulic components by skilled specialists. For example, such a system would also allow for achieving precise force control in active shock absorbers for a variety of vehicle and operating conditions.
Despite the need for a method and system that can adapt to varying driving mechanisms, no effective and efficient method is so far known. Thus, there is a need for a method and system for achieving force control in a multiplicity of hydraulic cylinders driven by external mechanisms with unknown parameters.